Method of obtaining a CdTe or CdZnTe single crystal and the single crystal thus obtained

ABSTRACT

The invention relates to the field of CdTe or CdZnTe single crystal production and to an improved solid-phase method of obtaining large CdTe or CdZnTe crystals having an excellent crystalline structure.

The present invention pertains to the obtaining of single crystals ofCdTe or CdZnTe. It relates to an improved method for obtaininglarge-size CdZnTe or CdTe crystals having excellent crystallinestructure.

Cadmium telluride or CdTe can be considered as the major representativeof the type II-VI semiconductor family.

Over approximately the last forty years, CdTe has aroused considerableinterest and has found numerous industrial applications. It has givenrise to several international conferences, and its history was given inthe monograph by K. Zanio (Semiconductors and Semimetals, Vol.13,Cadmium Telluride, Academic Press 1978). CdTE has a unique set ofproperties making it highly suitable for numerous applications. Itsforbidden bandwidth of 1.5 eV, right in the centre of the solarspectrum, makes it an ideal material for photovoltaic conversion, andnumerous companies market CdTe-based solar cells. Its high averageatomic number of 50, its wide forbidden bandwidth and its good electrictransport properties are adapted to nuclear detection. Its highelectro-optical coefficient combined with its low absorption coefficientare used for the production of high-performance electro-opticalmodulators and photorefractive devices. CdTe has two types ofconductivity, n and p, enabling its use for the production offield-effect diodes and transistors. Semi-magnetic conductors, such asCdMnTe, also offer advantageous properties such as gigantic Faradayrotations making them very attractive for optic insulators.

CdTe is also used at industrial level in the form of its ternary alloyCdHgTe, one of the major materials for infrared detection, and CdZnTe,which is used as substrate for epitaxial depositing of HgCdTe layers,and as nuclear detector, this latter application being of particularinterest especially for medical use.

Two applications currently dominate the industrial development of CdTe:

-   -   CdTe and more particularly its alloy with 4% zinc, is very        widely used as substrate for epitaxial depositing of HgCdTe        layers, a leading material in the area of infrared detection,        especially in the two bands of wavelength 3-5 μm and 8-12 μm.        Several industrial companies produce and market substrates of        Cd_(0.96)Zn_(0.04)Te and the tendency is towards the fabrication        of substrates of increasing size for the subsequent fabrication        of focal plane arrays for very large-size IR detectors.    -   the binary compound CdTe, and its alloy CdZnTe with low        concentrations of zinc, is currently the subject of intensive        research in the area of nuclear detection for which it offers an        enormous potential, especially in medical applications, both for        X-ray detection with medical radiography applications, and in        gamma detection with medical gammagraphy applications (gamma        camera). The much superior detection rapidity of the CdTe        semiconductor compared with current scintillation detectors        allows for real time imaging, highly advantageous for the        observation of body parts in movement such as the heart. Also,        CdTe detection is conducted at ambient temperature and not at        the temperature of liquid nitrogen as for scintillation        detectors, hence lower operating costs and a very substantial        lower investment in equipment, and the possibility of producing        mobile equipment.

Both epitaxial substrate applications and applications relating tonuclear detection necessarily involve the use of single crystals, verydifficult to prepare up until now. The difficulty in producing singlecrystals is currently holding back the development of theseapplications. This is why considerable efforts have been devoted overthe last forty years to preparing CdTe single crystals.

Growth techniques for CdTe single crystals can be briefly recalled. Theyare divided into several categories:

-   -   growth from a liquid, whether stoichiometric or        non-stoichiometric, whose limits are detailed further on;    -   vapour phase growth, either by sublimation or by chemical        transport. These techniques in which the ionicity of the CdTe        bond also has a very negative influence, are limited by the        uncertainty of obtaining single crystals and by very slow growth        rates, unfit for industrial growth.

Given its melting point of 1092° C., lower than the softening point ofsilica, liquid phase growth nonetheless remains the most frequently usedfor CdTe, and it is this technique which is essentially used atindustrial level in its vertical, horizontal or pressure Bridgmanvariants.

The preparation of CdTe single crystals raises numerous technicaldifficulties of which some are detailed below.

Phenomena of pre- and post-transition are known, below and above themelting point of CdTe, in both liquid and solid states. Pre- andpost-transition phenomena are due to the ionic nature of the Cd—Techemical bond. This ionic nature leads to highly associated CdTe liquidsin the vicinity of the melting point, which leads to the presence ofhighly organized particles influencing the nucleation process and growthkinetics.

In the solid state, and again close to the melting point, phasetransitions reported by numerous authors also markedly complicate thecrystallogenesis of CdTe in liquid phase.

In addition, earth gravity field effects cause convection phenomenawhich are difficult to control. To overcome this, recourse may be had tomicrogravity or macrogravity crystal growth techniques for liquid phaseand vapour phase methods, whose cost is evidently prohibitive.

In the methods of the state of the art, and in particular in methodsusing the BRIDGMAN method to obtain CdTe crystals, it was sought toreduce the influence of convection phenomena by growing crystals undermicrogravity, in particular during journeys into space under conditionsof weightlessness, and under macrogravity in centrifuges.

The numerous technical drawbacks cited above in the obtaining of CdTe orCdZnTe single crystals in liquid phase or vapour phase have beenresolved by the invention.

It has been shown in the invention that a CdTe or CdZnTe single crystalhaving excellent single crystalline structure could be obtained using amethod conducted in solid phase.

The subject of the invention is a method for obtaining a CdTe or CdZnTesingle crystal in which x lies between 0 and 0.2, characterized in thatit comprises the following steps:

-   -   a) obtaining a source material of Cd_(1-x)Zn_(x)Te having a        composition close to that corresponding to congruent        sublimation, either by crystallizing liquid Cd_(1-x)Zn_(x)Te        using Bridgman's horizontal technique under partial cadmium        pressure to adjust stoichiometry, or by annealing a        Cd_(1x)Zn_(x)Te material pre-synthesized at 800-900° C. in an        ampoule with end capillary maintained at ambient temperature.    -   b) obtaining a polycrystalline block of Cd_(1-x)Zn_(x)Te in        which x lies between 0 and 0.2 by sublimation of a        stoichiometric solid source material of Cd_(1-x)Zn_(x)Te in a        sealed chamber at a temperature of the chamber zone in which the        source material is located of between 900° C. and 1000° C., the        difference in temperature ΔT between the zone of the source        material and the zone of crystal deposit lying between 30° C.        and 50° C.    -   c) obtaining a Cd_(1-x)Zn_(x)Te single crystal by        re-crystallization of the polycrystalline block obtained at step        b), by annealing under isothermal conditions under partial Cd        pressure lying between 4.10⁵ and 6.10⁵ Pa and at a temperature        of between 1000° C. and 1060° C. for a time of between 50 and        200 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a photograph of a single crystal obtained according toexample 1.

FIG. 2 shows the results of a retro-diffused-electron-diffraction test.

The symbols “Cd”, “Te” and “Zn” respectively represent the atoms ofcadmium, telluride and zinc.

The method for obtaining a Cd_(1-x)Zn_(x)Te single crystal is conductedat a lower temperature than the melting point of the alloy.

Under these temperature conditions, the phenomena of phase transitionare not observed which occur when implementing liquid phase growthmethods for crystalline blocks described in the prior art.

In addition, the conditions of the above method enable the growth ofpractically stoichiometric crystals, unlike conventional methods inwhich adjustment is more difficult. The source material used for thesublimation step is prepared so that its composition allows congruentsublimation and hence a high transport speed, i.e. with a compositionclose to stoichiometry.

Another determinant technical advantage of the method of the inventionis that this method makes it possible to overcome field effects of earthgravity which cause uncontrolled convection phenomena.

According to a first preferred embodiment of the above method, the valueof x in the formula Cd_(1-x)Zn_(x)Te is equal to zero and the singlecrystal obtained is a CdTe single crystal.

In a second embodiment of the above method, the value of x in theformula Cd_(1-x)Zn_(x)Te is different from zero, and the single crystalobtained is a CdZnTe alloy single crystal.

The parameter x advantageously lies between 0.005 and 0.05, preferablybetween 0.01 and 0.05, and further preferably is 0.04, in particularwith a view to obtaining epitaxy substrates for HgCdTe.

Step a) of the Method

This step consists of producing a source material whose composition isclose to stoichiometry intended for the second sublimation step.According to the method, this source material is obtained by reaction ofCd and Te elements, and optionally Zn in stoichiometric proportion, thenadjustment of stoichiometry either by crystallization of the melt usingBridgman's horizontal method under partial cadmium pressure so as tocontrol stoichiometry difference, or by annealing the synthesizedmaterial at a temperature of between 800 and 900° C. in a capillaryampoule in which any excess of one of elements Cd or Te is condensed.

Step b) of the Method

This step consists of forming an ingot of polycrystalline CdTe bysublimation of a source material whose composition is close tostoichiometry. As results directly from the characteristics of themethod, a temperature gradient is set up in a sealed chamber, thesurface of the source material being maintained at sufficienttemperature to cause sublimation of Cd_(1-x)Zn_(x)Te, and the depositzone being maintained at a lower temperature.

The temperature of the chamber zone in which the source material islocated, which lies between 900° C. and 1000° C., was chosen so as to besufficiently high to promote sublimation from the solid source materialwhile remaining lower than the melting point of CdZnTe.

Advantageously, the temperature of the source material zone lies between940° C. and 960° C., preferably between 945° C. and 955° C. It isfurther preferably approximately 950° C.

So as to conduct sublimation under optimum conditions, the difference intemperature ΔT between the zone of the source material and the zone ofcrystal deposit is between 30° C. and 50° C.

With a temperature difference ΔT lower than 30° C., it was observed thatthe structure obtained tended to be a large-grain structure which isless favourable for the subsequent re-crystallization step. It isrecalled that the energy accumulated at the grain joints forms thedriving force for re-crystallization. This energy is greater the smallerthe grain size and consequently the greater the joint surface.

With a temperature difference ΔT greater than 50° C., pores and holeswere seen to occur in the ingots obtained.

Preferably, the distance between the evaporation surface of the sourcematerial and the zone of crystal deposit lies between 10 and 20 cm, andfurther preferably between 10 and 15 cm.

The optimal conditions for sublimation to form a “polycrystalline” blockat step b) of the method are reached through a combination of parametersΔT and the distance between the surface of the source material and thedeposit zone.

One advantageous parameter of the method according step b) is thethermal interference set up at the growth interface, intended to avoidexcessive grain growth during deposit. This thermal interference isachieved by setting up an oscillating temperature schedule in thedeposit zone.

Without introducing this thermal interference, the initial crystallitesdeposited during step b) form nuclei which grow in time to form singlecrystal structures, which is precisely what it is sought to avoid atthis stage of the method. To obtain a polycrystalline ingot or block oncompletion of step b) of the method, thermal interference is introducedwhich causes the continual formation of new nuclei over time.

Preferably, a sinusoidal thermal interference is set up which leads tothe formation of new nuclei at each temperature oscillation cycle.

Preferably, for a ΔT value of 30° C., a variation in temperature havinga magnitude of 10° C. is set up within the chamber so that value ΔTvaries from 20° C. to 40° C., for example by adequate programming of theheating means. The sinusoidal period may vary, in particular in relationto technical constraints related to the heating means and to the thermalinertia of the source material itself and the duration of step b) toobtain a polycrystalline ingot or block. By way of illustration, asinusoidal period of between 10 and 30 minutes is preferred, betterbetween 15 and 25 minutes and preferably between 18 and 22 minutes,further preferably of 20 minutes.

In order to perform the method of the invention under optimalconditions, in particular under optimal conditions of thermodynamics andhydrodynamics, use is made of a sealed silica chamber in the shape of atube preferably having flat ends. At step b) of the method, the geometryof the ampoule is of importance. In particular, the surface of theprepared polycrystalline block determines the surface of the singlecrystal ingot or block which is the end product of the method. Hence thegreater the surface of the polycrystalline block, the greater thesurface of the end single crystal ingot or block.

Obtaining a large-size polycrystalline block may advantageously beachieved by using a chamber, an ampoule for example, having a flat endfrom whose surface the polycrystalline block is formed. With acylindrical chamber, such as an ampoule, the diameter of the chamberdetermines the surface of the polycrystalline block formed at step b)and hence also the surface of the end single crystal block.

This geometric characteristic of the chamber used at step b) of themethod amounts to an additional difference of the method of theinvention compared with known methods in the prior art, in which achamber with a tapered tip is preferably chosen, so as to promote theformation of a single nucleus, by deposit, from which a single crystalnetwork is to be formed.

According to the invention, at the end of step b), polycrystallineblocks were obtained having a diameter of up to 50.8 mm (up to 2 inches)in diameter.

The duration of step b) is conventionally several hours, untilcompletion of the formation of a polycrystalline ingot or block. It isgenerally between 5 hours and 48 hours, advantageously between 10 hoursand 36 hours, preferably between 15 hours and 30 hours and is optimally24 hours.

Step c) of the Method

At the end of step b) of the method, a polycrystalline block ofCd_(1-x)Zn_(x)Te is obtained having the desired stoichiometry betweenthe elements Cd, Zn and Te.

In addition, the polycrystalline blocks obtained at the end of step b)of the methods are free of pores, also called “microcavities”, whichcould be found in single crystals after re-crystallization and whichwould substantially affect their quality and properties.

At step c) of the method, the Cd_(1-x)Zn_(x)Te single crystal isobtained by re-crystallizing the polycrystalline block obtained at stepb).

Re-crystallization annealing operations are conducted isothermalfashion.

The partial pressure of Cd of between 4·10⁵ and 6·10⁵ Pa makes itpossible to avoid decomposition of the material.

The temperature of step c), which lies between 1000° C. and 1060° C., isboth sufficient to obtain conversion of a polycrystalline network into asingle crystal network while remaining lower than the melting point ofCdTe or CdZnTe.

At a temperature of less than 1000° C., the annealing time may becomemuch too long and costly, temperature being an essential parameter forgrowth kinetics.

At a temperature of over 1060° C., this step would be too close to themelting point and to phase pre-transitions reported in the literature.

The duration of step c) lies between 50 and 200 hours so as to allowcomplete conversion of the polycrystalline network into a single crystalnetwork.

The geometry of the chamber in which step c) of the method is performed,is not determinant for the results to be achieved. It arises from thecharacteristics of the method that the single crystal ingot or block isformed directly from the polycrystalline ingot or block prepared at stepb), and that all that is required is that the chamber used at step c)should have a volume which is able to receive the polycrystalline blockprepared at step b).

In best preferred manner, the sealed chamber is an ampoule whose innerdiameter is no more than 50.8 mm (no more than 2 inches).

It has been shown in the invention that re-crystallization step c) toobtain a single crystal is not inhibited by the presence of zinc in CdTewhen zinc is present in CdTe at concentrations of 20% atoms or less.

Also, it has been shown in the invention that the concentration of zinc,measured in the single crystal material after re-crystallization, isequal to the concentration of zinc in the source material. Withoutwishing to be bound with any theory, the applicant believes that thisspecific characteristic of the method of the invention indicates thatthe phenomenon of zinc segregation in CdZnTe, which is observed inliquid phase techniques or gaseous phase techniques of the state of theart, does not occur when implementing the method of the invention, or issignificantly reduced.

The absence of the zinc segregation phenomenon in CdTe when applying themethod of the invention amounts to an important technical advantagesince the segregation of zinc, in particular in liquid phase crystalgrowth techniques, is a major technical disadvantage drasticallylimiting production yields of end products such as substrates anddetectors.

Therefore the method for obtaining a Cd_(1-x)Zn_(x)Te single crystalaccording to the invention, can also be characterized in that the sourcematerial is obtained using the following method:

-   a) preparing a starting source material according to one of the    following methods:    -   i) reaction of Cd and Te, and optionally Zn, in stoichiometric        quantities, in liquid phase and at high temperature;    -   ii) reaction of Cd and Te, and optionally Zn, in stoichiometric        quantities in vapour phase;-   b) adjustment of Cd/Te stoichiometry in the source material obtained    at step a) using one of the following methods:    -   iii) annealing the source material at a temperature of between        800° C. and 900° C. in an ampoule with capillary, condensing any        excesses of one of elements Cd and Te;    -   iv) crystallizing the source material using Bridgman's        horizontal method, under partial Cd pressure (Horizontal        Bridgman growth of large high quality Cd_(1-y)Zn_(y)Te        crystals, P. BRUNET, A. KATTY, D. SCHNEIDER, A.        TROMSON-CARLI, R. TRIBOULET, Mater Sci. Eng. B16 (1993)44)

The Cd_(1-x)Zn_(x)Te single crystal which forms the end product of themethod of the invention is a fully single crystal, which is rarelyobtained, and at all events not in reproducible manner with any of thevapour phase or liquid phase methods described in the state of the art.

In particular, the method of the invention has already made it possibleto obtain single crystals with a large surface area, up to 4 cm². Singlecrystals of 20 cm² are currently under preparation.

For CdTe single crystals, it has been shown in the invention that thesesingle crystals have pseudo-Kikuchi lines with up to 4^(th) orderresolution. Pseudo-Kikuchi lines are electron-channelling patterns seenunder scanning electron microscopy.

It was also shown that the CdTe single crystals obtained with the methodof the invention showed rocking-curve widths at mid-height under doubleX-ray diffraction of less than 30 arc seconds, which demonstrates theirexcellent crystallographic quality.

In addition, the CdTe or CdZnTe single crystals obtained in reproduciblemanner with the method of the invention have a growth axis alongdirection (111).

This characteristic is particularly advantageous for the production ofepitaxy substrates whose (111) surfaces are preferably used atindustrial level.

The CdTe single crystals of the invention have polar (111) planes solelymade up of telluride atoms and planes solely made up of cadmium atoms,which considerably facilitates crystal cutting operations so as toobtain crystal wafers cut along the growth axis of the ingot. Thetelluride planes and cadmium planes, which are the “A” and “B” faces ofthe crystal wafer, are perpendicular to the <111> axis.

The invention also relates to a Cd_(1-x)Zn_(x)Te single crystal in whichx lies between 0 and 0.2, characterized in that:

-   -   (i) it has pseudo-Kikuchi lines with up to 4^(th) order        resolution,    -   (ii) it has mid-height rocking-curve widths of less than 30 arc        seconds.

The invention also concerns the use of a Cd_(1-x)Zn_(x)Te single crystalsuch as defined above as substrate for the epitaxial deposit of a layeror plurality of layers of HgCdTe.

It also pertains to the use of a Cd_(1-x)Zn_(x)Te single crystal such asdefined above for the fabrication of an X-ray detector device or gammaray detector device.

It also concerns the use of a single crystal according to the inventionfor the fabrication of an electro-optical modulating device or aphotoreactive device.

It also relates to the use of a single crystal such as defined above forthe fabrication of a diode element or transistor element.

For the various uses of a Cd_(1-x)Zn_(x)Te single crystal according tothe invention, persons skilled in the art may advantageously refer tothe work by K. Zanio (cited above).

The invention is also illustrated, without being limited to, thefollowing examples.

EXAMPLES Example 1 Preparation of a CdTe Single Crystal According to theInvention

A. Material And Methods

A.1 Preparation of a CdTe Starting Material

The starting material is taken from a CdTe ingot derived from Bridgman'shorizontal method under controlled partial cadmium pressure to adjuststoichiometry. The partial pressure of cadmium corresponds to a coldpoint in the chamber of 780° C. This experimental protocol is describedin the following article: Horizontal Bridgman Growth of Large HighQuality Cd_(1-y)Zn_(y)Te Crystals, P. BRUNET, A. KATTY, D. SCHNEIDER, A.TROMSON-CARLI, R. TRIBOULET, Mater. Sci. Eng. B16 (1993)44.

Alternately the starting material may be prepared using Bridgman'svertical method with or without partial cadmium pressure control, thenannealed at 900° C. in a capillary ampoule whose end is maintained atambient temperature to adjust the composition to a value correspondingto congruent sublimation.

A-2 Preparation of a Polycrystalline Block of CdTe

The starting material, typically 60 g CdTe, is loaded into an ampoulewith an inner diameter of 2 cm, subsequently sealed under a secondaryvacuum. The ampoule, graphited at its two ends, is placed in a reactorwith eight heating zones enabling a thermal profile to be obtained whendesired so that the feed is at 950° C. and the cold point, 12 cm awayfrom the feed, at 920° C. Thermal oscillation of a magnitude of 10° C.is set up using Eurotherm 906P regulators/programmers at the growthinterface. The period of this oscillation is approximately 20 minutes.The tube is held fixed in this reactor for 24 h, a sufficient period forthe entirety of the source material to be transported to the cold point.

A-3 Obtaining a CdTe Single Crystal

After removing the above-mentioned ampoule, an operation readilyperformed due to the presence of graphite on the ampoule walls, thepolycrystalline ingot is placed in an ampoule that is also graphited.This ampoule, after sealing in a secondary vacuum, is placed in theisothermal part of a horizontal furnace at 1060° C. The ampoule alsocontains a quantity of cadmium corresponding to a partial pressure of5×10⁵ Pa at 1060° C. for the volume under consideration. This quantityof cadmium is determined taking into account the perfect gas equation,PV=nRT. The ampoule is maintained at this isothermal phase for 120 h.

B. Results

Analysis of the Characteristics of the Single Crystal Obtained.

The entirety of the ingot subjected to re-crystallization proves to be asingle crystal. A rocking-curve width at mid-height, under dual X-raydiffraction, of 30 arc seconds and pseudo-Kikuchi lines in electronchannelling under scanning electron microscopy, demonstrate itsexcellent crystallographic quality. In addition, the source materialobtained after sublimation step b) has a texture, as analysed underelectron back scattering diffraction (EBSD), such that the generalorientation of the growth axis of the crystals is close to (111) towithin a few degrees.

This orientation is maintained in the crystals obtained afterre-crystallization step c). The large face of the crystals obtained istherefore the (111) plane, which is highly favourable when cuttingCdHgTe epitaxy substrates for which this is precisely the requiredorientation.

FIG 1. is a photograph of a single crystal obtained according toexample 1. On this photograph, one can see that the final product of theprocess of the invention is a structure which is exclusively a singlecrystal, in which polycrystalline crystals are absent. Moreparticularly, the uniform aspect of the surface of the section of thecrystalline block, shown in FIG. 1, demonstrates the absence of grainboundaries, and therefore the absence of polycrystalline structures.Furthermore, the scale graduated in millimeters, at the lower part ofFIG. 1 shows that the surface of the final product, which is a singlecrystal obtained by the process of the claimed process, is about 4 cm 2(2 cm ×2 cm).

FIG. 2 represents the results of a test of retro-diffused-electrondiffraction (EBSD). One can see from FIG. 2 that the crystalline blockobtained by the process of the invention is exclusively a singlecrystal. Indeed, the EBSD analysis is realised by scanning a beam ofelectrons on all the surface of the crystal according to the invention.The uniform blue colour demonstrates that the general orientation of thegrowing axis of the crystals is closed to the axis (111) and that thecrystalline block is exempt of grain boundaries.

1. A method for obtaining a Cd_(1-x)Zn_(x)Te single crystal, in which xis a value from 0 to 0.2, which comprises the following steps: a)obtaining a source material of Cd_(1-x)Zn_(x)Te having a compositionclose to that corresponding to congruent sublimation, either bycrystallization of liquid Cd_(1-x)Zn_(x)Te using Bridgman's horizontaltechnique under partial cadmium pressure to adjust stoichiometry, or byannealing a Cd_(1-x)Zn_(x)Te material pre-synthesized at 800-900° C. inan ampoule with end capillary maintained at ambient temperature, b)obtaining a polycrystalline block of Cd_(1-x)Zn_(x)Te in which x liesbetween 0 and 0.2 by sublimation of a stoichiometric solid sourcematerial of Cd_(1-x)Zn_(x)Te in a sealed chamber at a temperature of thechamber zone in which the source material is located at between 900° C.and 1000° C., the difference in temperature ΔT between the zone of thesource material and the zone of crystal deposit lying between 30° and50° C. c) obtaining a Cd_(1-x)Zn_(x)Te single crystal byre-crystallizing the polycrystalline block obtaining at step b), byannealing under isothermal conditions under partial Cd pressure ofbetween 4·10⁵ and 6·10⁵ Pa and at a temperature of between 1000° C. and1060° C. for a time of between 50 and 200 hours.
 2. A method as in claim1, wherein the Cd_(1-x)Zn_(x)Te single crystal is a CdTe single crystalwhen x=0.
 3. A method as in claim 1, step b) of which the temperature ofthe source material zone lies between 940° C. and 960° C.
 4. A method asin claim 1, in step b) of which the distance between the evaporationsurface of the source material feed and the deposit zone lies between 10and 20 cm.
 5. A method as in claim 1, in step b) of which thecondensation surface is subjected to a sinusoidal temperaturefluctuation of a magnitude in the order of 10° C. and whose period isapproximately 20 minutes.
 6. A method as in claim 1 wherein the sourcematerial is obtained by the following method: a) preparing a startingsource material according to one of the following methods: i) reactionof Cd and Te, and optionally Zn, in stoichiometric quantities, in liquidphase and at high temperature; ii) reaction of Cd and Te, and optionallyZn, in stoichiometric quantities, in vapour phase; b) adjustment ofCd/Te stoichiometry in the source material obtained at step a) using oneof the following methods: iii) annealing the source material at atemperature of between 800° C. and 900° C. in an ampoule with capillary,condensing any excesses of one of elements Cd and Te; iv) crystallizingthe starting source material using Bridgman's horizontal method, underpartial Cd pressure to permit stoichiometry adjustment.
 7. A method asin claim 3 wherein the temperature of the source material zone isbetween 945° and 955°0 C.
 8. A method as in claim 7 wherein thetemperature of the source material zone is approximately 950°.
 9. Amethod as in claim 4 wherein the distance between the evaporationsurface of the source material feed and the deposit zone lies between 12and 15 cm.